The present invention relates to energy storage and in particular to a thermal energy storage system utilizing moving bed heat exchangers.
Electricity produced by an electric power generating plant must be consumed immediately or it is lost. The demand for electricity from such a plant is not constant but varies throughout the day. Therefore electric power generating plants must be designed to operate over a range of production levels and moreover, to be capable of producing enough electricity to satisfy peak demands.
Designing the plant to peak load capacity is inherently uneconomical in that plant construction costs are proportional to capacity. Ideally the plant could be constructed at average load level capacity thereby avoiding the higher construction costs for peak capacity. In order to do this, peak demands must be met by some supplemental source. Present available sources of supplemental energy for use during peak demand periods include diesel engines, additional fossil fired steam turbine-generators, and stored energy.
The present invention is an energy storage system designed to supply peak demand energy for an electric generating facility. According to the invention, the electric generating plant operates at a constant average load fuel consumption rate. During slack demand periods, when electricity is consumed at less than the average rate, the storage system is charged by the surplus energy generated. During peak demand periods, when electricity is consumed at higher than the average rate, the storage system is tapped to enable production of the needed additional electricity while the main energy source, eg. fossil boiler or nuclear reactor, continues to operate at a constant rate.
Of the energy storage systems available in the prior art, pumped hydro is the most feasible. In the pumped hydro system surplus electricity generated during slack demand periods, is used to pump water to higher elevation, usually into a dammed lake, where it is retained. During high demand periods, the water is released to flow down through hydro turbines, thereby generating needed electricity. Unfortunately, pumped hydro energy storage is limited in application by a paucity of acceptable sites for building dams and is further limited by opposition from environment conscious groups opposing dam construction.
Another storage system concept available to steam cycle electric generating plants is the removal of thermal energy directly from the steam cycle during slack demand periods to be stored for later utilization during high demand periods. One such system known in the prior art diverts steam from the steam cycle to heat oil. Hydro carbons such as oil have heat storage properties superior to those of water. However, oil typically loses its integrity if heated beyond 650 degrees Fahrenheit and is limited thereby. Steam temperatures in modern fossil fired plants can exceed 1,000.degree. F. It is desirable to heat the energy storage medium to the highest temperature possible to maximize heat recovery efficiency. The hot oil system is therefore inherently limited in that it cannot be used at the highest temperatures possible because of its loss of integrity. Moreover, no known liquid of suitable cost maintains its integrity at 1000.degree. F. at moderate pressures. Solids, however, are superior heat storage mediums in that they maintain integrity at elevated temperatures and at low pressure. Also, more heat per unit volume can be stored in solids than in liquids because of the greater density of the solid notwithstanding a possible lower specific heat capacity.
One known system utilizing a fixed solid for energy storage diverts hot fluid through holes in a solid block during slack demand periods thereby charging the block, or bed ie., raising the temperature of the solid. During high demand periods, the block is used to heat cooler steam or water thereby discharging the block, ie., returning the energy to the system. This system may be used at high temperatures, however, it has an inherent disadvantage. The temperature of the block and heat transfer fluid during discharge thereof is not constant but rather is decreasing. As the temperature of the block decreases, the efficiency of heat transfer likewise decreases, resulting in a declining energy yield.
The present invention uses a bed of free-flowing refractory particles for heat transfer and heat storage and provides both a solid heat storage medium for use at high temperature and a constant temperature heat source during system discharge.
Apart from prior art in the field of heat storage technology, prior art exists in the field of free-flowing solid granules or microspheres as a mechanism for heat storage and transfer for advanced energy source power reactors. A typical description of research in this field is described in the paper titled, "Moving Bed Heat Transfer for Advanced Power Reactor Applications" published by Mr. D. C. Schluderberg and Mr. T. A. Thornton at the Miami International Conference on Alternative Energy Sources, in Miami Beach, Fla., Dec. 5 through 7, 1977. This paper reported the result of some tests conducted with gravity flow moving beds of free-flowing microspheres over spiral tubing in a small laboratory-scale apparatus.
A number of proposals also were described in this paper for recirculating particles from a hot reservoir through a steam generator to a cold reservoir for subsequent recirculation by means of an Archimedes spiral lift tube arrangement to the heat source and back to the hot particle reservoir.
The concept of particulate material as a heat transport mechanism also has been the subject of intensive research. The following collection of patents, for instance, are illustrative of the work that has been accomplished in this field of technology. U.S. Pat. No. 2,672,671, granted Mar. 23, 1954, for alumina-mullite pebbles is directed to a method of manufacturing high purity mullite-alumina pebbles that are capable of enduring severe conditions of cyclic thermal and mechanical shock. U.S. Pat. No. 2,644,799 granted July 7, 1953, for heat-exchange pebbles discloses the broad concept of a gravity-flow mass of pebbles for discharging stored heat. The cooled pebbles after heat discharge, are recirculated by means of a bucket or screw conveyor. U.S. Pat. No. 2,808,494, granted Oct. 1, 1957 for "Apparatus For Storing and Releasing Heat" shows a gas or oil fired system for heating an immobilized mass of powder or spheres in an heat storage apparatus.
U.S. Pat. No. 2,856,506, granted Oct. 14, 1958 for "Method for Storing and Releasing Heat" is a division of U.S. Pat. No. 2,808,494 and largely duplicates the disclosure in U.S. Pat. No. 2,808,494.
U.S. Pat. No. 3,615,187, granted Oct. 26, 1971 for "Process for the Production of Spherical Alumina-Silica Containing Solid Particles Which are Predominantly Mullite" is directed to production of the solid particles rather than to some application techniques.
U.S. Pat. No. 3,669,889, granted June 13, 1972, for "Granular Ceramic Heat Carrier Intended for Manufacture Thereof" describes a granular ceramic heat carrier for use in conjunction with chemical processes. The method for manufacturing these granules also is described in this patent.
In spite of the intensive research that has been applied to this general area of technology and clear energy conservation benefits of a practical adaptation of these techniques notwithstanding, there is, nevertheless, a continued need to come forward with more efficient moving bed and heat exchanger combinations. Further in this respect, there is a requirement to adapt this technology to practical heating power plant generation cycles in order to make this technology immediately available to the power utility industry.
These and other problems are satisfied to a large extent through the practice of the present invention wherein an arrangement of one or more silos are provided for absorbing heat from a mass of microspheres or other suitable particulate material. The microspheres absorb heat by flowing under gravitational force over heat exchanger tube bundles. Heat is provided illustratively from low and high-pressure hot water and/or steam in a conventional power plant.
In the present invention, the plant steam turbine output is modulated while fuel comsumption remains constant. In typical steam cycle electric generating plants, portions of the cycle steam are at various points extracted for heating feedwater flow from the condenser to the steam generator in order to increase cycle efficiency. In the present invention, during off-peak periods, the steam generator remains at full power while turbine output is lowered by increasing extraction steam flow at various points in the steam cycle thus lowering flow rate to the turbines. The flow extracted above the normal amount is used to heat the storage inventory of moving bed solids.
The extraction steam flows through a moving bed heat exchanger while moving bed materials flow down therethrough by gravity. In this way, by permitting the particulate matter to absorb from different temperature steam supplies within the power plant system a much more efficient heat storage and transfer system is provided. This system, moreover, is readily adaptable to the reheat steam cycle that characterizes many modern power generation plants today.
The hot moving bed solids are then stored in an insulated bin until needed. When peak demand power is required, the hot solids flow through a moving bed heat exchanger to heat feedwater flow directed therethrough, thereby allowing a reduction of extracted steam flow below normal and an increase in the amount of high pressure steam generated. This reduction in extracted steam and increase in high pressure and reheat steam flow results in increased turbine output and increased electricity generation.
In this manner, heat storage is used to vary plant power output above or below an established baseload while power input from the plant heat source is held constant at a level corresponding to the base load electrical output.
An object of the present invention is an energy storage and recovery system for storing excess energy generated by an electric power plant during slack demand periods and for recovering the stored energy to provide energy for producing supplemental electricity during peak demand periods.
A further object of the invention is a system yielding the foregoing advantages and which utilizes a moving bed of refractory particles for a heat transfer and storage medium.
Another object of the invention is a system yielding the foregoing advantages and which can be back fitted to existing power plants.
A further object of the invention is a system yielding the foregoing advantages and which can be used with fossil fueled plants or nuclear plants.
Other objects and advantages of the present invention will be readily apparant from the following description and drawings which illustrate the preferred embodiments of the present invention.